An opto-electronic integrated circuit (OEIC) often requires an optical waveguide to connect different portions of the circuit. The waveguide may extend between two widely separated optical or optoelectronic devices or may optically couple an optical fiber positioned at the chip edge to an interior device. One popular type of such an optical waveguide is a rib waveguide in which a rib of high refractive index is formed above the substrate of lower refractive index. Both the substrate and the air surrounding the sides of the rib act as optical cladding to confine light to the vicinity of the rib. A low-index upper cladding may be interposed between the rib and the air.
For complex OEICs, the waveguide should be single mode. That is, the waveguide should support only one propagating mode, the higher-order modes being leaky modes, which are suppressed over a substantial length of the waveguide. Thereby, mode dispersion arising from different propagation velocities of the modes does not occur and waveforms are maintained over long distances. These same reasons require long-distance optical fibers to be single-mode, i.e., support only the TEM.sub.0 mode. For single-mode operation, the cross-section of the waveguide is limited by the effective refractive index difference .DELTA.n between the rib material and some average of the refractive indices of the substrate and air. As a general rule, if the effective index difference increases, the cross-section must decrease if the rib is to support a single guided mode.
A large cross-sectional waveguide offers some advantages. It has reduced surface scattering. Its large size implies optical fields of large cross-section which efficiently couple to the relatively large optical fibers. However, a large waveguide suffers large bending loss and is too large for efficient coupling to low power opto-electronic devices on the OEIC.
In view of different constraints at different points on the waveguide, a tapered rib would be desirable in which the cross-section of the waveguide varies along its length. One method achieves single-mode operation in a tapered structure by shaping the rib to have a cross-section with a somewhat constant aspect ratio along its length. The lateral tapering can be easily achieved. The additionally required vertical tapering, although possible, introduces complex processing.
Shani et al. disclosed a tapered rib waveguide structure made with fairly standard processing in a technical article "Efficient coupling of a semiconductor laser to an optical fiber by means of a tapered waveguide on silicon, " Applied Physics Letters, volume 55, 1989, pages 2389-2391. Their waveguide structure transitioned between a large rib having a small effective index difference .DELTA.n with the cladding to a small rib with a large .DELTA.n. The small rib ended in a lateral taper and was there covered on its top and sides by the large rib. Although this structure avoids any complex vertical tapering, it does require redeposition (a regrowth) of the large rib over the substrate from which the small rib has been removed. It further requires separate lateral definition for the large and small ribs.